Kind of the ebb and flow of roleplaying, as it turns out. It's not much help, but I recently made an interesting discovery relating, at the very least, to my own behavior and that of a friend of mine: we've been doing 1x1 roleplays for quite a while, but have the similar issues of the game eventually petering out, perhaps not even that far in. As of a few months ago, however, we've moved on to playing tabletop-rpg games, but still on a text basis and still with a huge narrative focus (we reference character sheets maybe once or twice an evening) and because we keep a regular, weekly schedule, it's helped us stay on track and move forward. Granted, the format is a bit different as it's effectively instant chat roleplay (or as instant as 3 to 30 minutes per post can get), but I do find it interesting that we've made much more tremendous story progress so far than we usually manage and I'm considering using this format for all of our roleplay ideas moving forward. I feel a lot more confident about that type of game not fizzling out.
As said, that probably doesn't help here and is perhaps beside the point, but it's something that I felt excited about and it felt appropriate to share at this juncture.
On a more related note, I know I'm not super trustworthy in terms of being punctual, but I can offer to play a second character should there be a shortage at some point - playing a Blood Saint, it's tempting to have an actual hunter character as well.